At a meeting in 1986, the CFHT users' community identified a low spectral resolution multi-object spectrograph as one of the highest priorities for new instrumentation at CFHT. Although the original intermediate dispersion spectrographs constructed for the CFHT had high throughput and were of excellent optical and mechanical quality, they were designed for single slit observations with image intensifiers or electronographic cameras as detectors. The desire to observe many faint objects simultaneously and also the realization that the image quality at CFHT is routinely better than one arcsecond led to the design of the MOS/SIS spectrograph, a dual Multi-Object and Subarcsecond Imaging Spectrograph. It is composed, in fact, of two distinct spectrographs sharing a common interface with the telescope after the Cassegrain bonnette: one is optimized for multi-object observations over a large field (MOS), the other (SIS) for high spatial resolution observations incorporating rapid tip/tilt image stabilization similar to that very successfully used in the CFHT/DAO high resolution camera HRCam (McClure et al. 1989). Two movable 45 degree mirrors permit a feed to either MOS or SIS. At the present time, a switch from one spectrograph to the other during the same observing run is not allowed.
The MOS/SIS spectrograph was jointly designed and built by teams from the Dominion Astrophysical Observatory (DAO) in Victoria, theObservatoire de Paris-Meudon (OPM), the Observatoire de Marseille andCFHT. Work began on the designs in May 1988 and resulted in an instrument which saw its first light in July 1992. For several years from that time, MOS/SIS was the most popular instrument at CFHT. With the advent of wide-field imaging and regular AOB observations, it has taken a smaller, but still quite significant role in the observering schedule. MOS/OSIS have accounted for 25 - 30 night per semester over the past few semesters (1/2001).
SIS went through an important transformation in the second semester of 1996. With a change in optics extending the useful wavelength range to 2 µm, SIS became "OSIS".
Two movable 45 degree mirrors permit a feed to either MOS or OSIS. At the present time, a switch from one spectrograph to the other during the same observing run is not allowed.
While MOS is primarily designed for multi-aperture spectroscopy over a 10' x 10' field, OSIS gives a smaller field of view (3.6' x 3.6') with better sampling. Designed to be used with a 2048 x 2048 15 µm pixel CCD, OSIS still gives a good sampling of almost 2 pixels for aperture widths as small as 0.25 on a 21 µm pixel CCD. The designed wavelength range is 365 to 2000 nm, and typical efficiencies are between 60% and 80% for imagery and 50% to 60% for spectroscopy.
In addition, OSIS has its own precision acquisition and guiding system so that objects can be optimally centered on small apertures. This ensures that as much flux as possible from the target objects enters the apertures, and also allows narrower apertures to be used, thus reducing contamination from the sky or other sources of background light.
The addition of CFHT-IR to our collection of infrared detectors makes it possible to use OSIS in the near-IR. The switch between OSIS and OSIS-IR is just a change of the detector; the rest of the optical train of OSIS remains, including the grism wheel, filter wheel, and tip-tilt correction mirror. Since these components are warm, it is not possible to use OSIS-IR in the K-band. The enhanced background limits the practical observing to J and H bands. The CFHT-IR filter wheel, which is located in the dewar and therefore cooled, allows the use of cool, long-wavelength blocking filters (high-pass filters) to minimize the impact of the warm optics on J and H band observations. Four grisms are available for OSIS-IR, a high and low resolution grism for each of the J and H bands. The pixel scale for OSIS-IR is 0.211'' per pixel, with 18.5µm pixels.
To permit spectra of tens of objects to be obtained simultaneously, an on-line facility for producing precise masks from direct images with a laser drilling machine (LAMA) is available.